December 13, 2013

Realizing tunable conductivity in ‘tinker toy’ materials

Mark Allendorf (8600), left, Alec Talin and Francois Leonard (both 8656) measure the conductivity of a MOF device, shown on the monitor (upper left). The team has developed a technique that increases the electrical conductivity of one MOF by more than six orders of magnitude. (Photo by Dino Vournas)

Sandia researchers have devised a novel way to realize electrical conductivity in metal-organic framework (MOF) materials, a development that could have profound implications for the future of electronics, sensors, energy conversion, and energy storage.

A paper to appear in Science magazine, “Tunable Electrical Conductivity in Metal-Organic Framework Thin-Film Devices,” debuted in the Dec. 5 edition of Science Express. The paper — co-authored by a group of Sandia researchers and collaborators at the National Institute of Standards and Technology (NIST) — describes a technique that experiments show successfully increases the electrical conductivity of one MOF by more than six orders of magnitude.

“Fundamentally, this sheds enormous light on the conduction process in these materials,” says material scientist Alec Talin (8656), the paper’s lead author.

Materials researchers have considered MOF materials primarily for use in gas storage, drug delivery, and other conventional applications for porous materials. Their crystalline structure, which resembles molecular scaffolding, consists of rigid organic molecules linked together by metal ions.

This hybrid of inorganic and organic components produces an unusual combination of properties: nanoporosity, ultra-high surface areas, and remarkable thermal stability, which are attractive to chemists seeking novel materials that combine the superior performance of traditional inorganic semiconductors with the low cost and ease of fabrication typical of conducting organic polymers.

Mark, a chemist and MOF expert who called the research findings the most exciting development in his 28-year Sandia career, likens them to “tinker toys” for chemists.

“When you imagine the ‘tinker toys’ we played with as children, you recall they are essentially wooden balls with holes that you can link together with sticks,” Mark says. “MOFs work the same way, only you substitute metal ions for the balls and organic molecules for the sticks.”

The resulting open space within the scaffolding can be filled with guest molecules, which gave Alec the idea to use the pore to make the MOFs electrically conducting.

“Importantly, MOFs possess a characteristic of molecules that allows us to adapt their properties to a specific application. We can perform chemistry on them, unlike traditional inorganic electronic materials, such as silicon and copper,” says Alec. Molecules, he says, represent the “ultimate, small-scale unit” at which electronic devices can be made. They are so difficult to manipulate and organize, however, that practical “molecular electronics” have not been realized. “How you connect to molecules, where you place them — those issues have consistently perplexed materials scientists,” says Alec.

The power of empty space

So he considered a different approach. “With MOFs, we can get around this problem by using the nanopores to organize molecules. The trick is to pick the right kind of molecule, so that it binds to and interacts with the entire framework.” Some MOFs, says Alec, have empty holes in the tinker-toy balls that can bind molecules that infiltrate the pores.

“This isn't like silicon, which can’t change its electrical properties,” Alec says. “You can add tiny amounts of dopants to silicon or introduce other impurities, but with our approach, you suddenly have the potential to tailor the material to achieve exactly the properties you want. This is the beauty of molecular electronics.”

To test their hypothesis, Sandia and NIST researchers added a molecule known as tetracyanoquinodimethane, or TCNQ, to their framework. First, they took a substrate with platinum electrodes patterned on its surface and coated it with a thin film of the MOF. The substrate was then dipped in a solution containing the TCNQ molecule, which they knew would seep into the MOF’s tiny pores. The MOF film containing the TCNQ bridged the electrode connection points, which then could be connected to a current meter for measuring.

“Frankly, I thought it would never work,” says Mark. “But that’s the great thing about science: Being wrong can be a good thing.”

The results are in, and they are good

The research team found that the MOF materials were conducting, though at relatively small quantities at first. “It was clear that something good was happening, so we were very excited,” says Mark.

The experiment was repeated several times with slight but important improvements in film quality achieved by optimizing the laboratory fabrication process.

“Conditions matter, and we had to be very deliberate in how we prepared the framework to accept the guest molecule,” says Mark. Removing the water and excess solvent from the film is no trivial matter, he says. The research team fine-tuned the process over the course of several months and, in doing so, began to see large leaps in electrical conductivity.

“The increase was massive,” says Alec. The conductivity in the material, he says, is now a million times higher than that of the starting material, and a thousand times higher than anything previously reported using a metal-organic framework.

The researchers plan to patent their approach and also hope to land additional funding to experiment with other guest molecules.

Keeping up with Moore’s Law and other applications

“The overwhelming success of this project opens a whole new way to design electrically active materials,” says Alec. Organic materials, he notes, offer low costs and mechanical flexibility. “There are probably hundreds of potential applications for this work that come into play, such as breath analysis and microelectronics,” he says.

The ability to make smaller and faster electronic devices to keep up with Moore’s Law has always been a motivator in the field of molecular electronics, Mark says. MOFs have the potential to create molecular electronic devices on the scale of their pore dimensions, or approximately 1 nm.

Solar technology is another potential application, says Mark, and DOE’s SunShot Initiative has funded some of the initial research. “With electrically conducting MOFs, we might very well be able to combine the high efficiencies achievable with traditional inorganic thin film materials such as polycrystalline Si with the low cost and flexibility aspects of organic photovoltaics,” he says.

“Our next step needs to be the exploration of other hosts and guest molecules,” says Alec. “We’d like to experiment with different MOF structures and different organic molecules to see if new behavior emerges. We want to see where this new learning takes us.”

Sandia conducts B61-11 pull down surveillance test; first in years

This is a composite of three images captured from high speed video showing the test unit in free flight as it approaches and penetrates a concrete target. The images show water vapor (from the light rain that was falling at the time of the test) surrounding the test unit. The square speckles on the test unit and the concrete target provide a random pattern that is used in digital image correlation algorithms to calculate test unit motion in 3-D using special equipment.

A dozen Sandians erupted in applause as they watched on video monitors while a rocket-driven B61 — a nuclear weapon, minus its nuclear components — rammed through a target at a test range in Sandia’s first such impact test in seven years.

“It’s been a while,” said Science and Technology Div. 1000 VP Duane Dimos, who recalled being told the test of the B61 gravity bomb was up next when he took his previous job as Center 1500 director in March 2010. “Thank you all,” he told the team.

Preliminary results look good

The test unit will be disassembled and final data analyzed, but data available the day after the test showed Sandia’s components worked as expected. The test weapon was equipped with instruments to measure component performance and velocity as it slammed into the target. Org. 2110 senior manager Patrick Sena says preliminary data showed the test met the requirements of the worst-case conditions the B61 is expected to meet with high reliability.

“One of the main purposes of the stockpile is deterrence, and one important way to ensure deterrence is to have a successful surveillance test that shows our systems work,” Patrick says.

The test unit was cooled to an internal and external temperature far below 0 degrees Fahrenheit. It had to be de-iced twice before the test as falling rain froze on the casing.

With weather conditions worsening, test officials decided to go ahead as soon as technicians had everything ready. As a result, the test went off 15 minutes earlier than scheduled on Nov. 20.

Dennis Miller, senior manager of the Validation and Qualification Group (1530) that conducted the pull-down test, sent an email a few minutes after the test, saying preliminary information was all positive. “The test unit was released successfully, and there is a big hole in the concrete target,” he wrote.

Emails of congratulations came in from around Sandia, including from Deputy Labs Director and Executive VP for National Security Programs Jerry McDowell (“Early congratulations to all.”) and Div. 2000 VP for Weapons Engineering and Product Realization Bruce Walker (“Fantastic news. My congratulations to the team.”)

“It’s been a long time coming,” Dennis said shortly before the test. “There’s been a lot of planning and anticipation.”

Data is precious

Sandia’s annual surveillance program for each weapon type consists of flight tests, lab tests, and component and material tests.

Flight tests, the most realistic, subject the test unit to shock, vibration, temperature extremes, rotation, weather, and so on. “Data from flight tests is precious because it is from a single-shot device,” Patrick says.

Sandia had worked toward the test since March 2010, the end of a hiatus on all such testing that was prompted by an October 2008 accident at Sandia’s 10,000-foot sled track. In the last three years, Sandia rebuilt its ability to run the tests, including reconstructing the firing set and safety systems that ignite the rocket motors and explosives-driven cable-cutting systems at the aerial cable facility, acquiring new rocket motors, and putting a strong emphasis on safety and technical performance assurance for the test. In addition, it rebuilt a team. Dennis says nearly all the employees and contractors who set up November’s test had never participated in a pull-down test before.

“This is a very complicated test,” he says. “There were about 60 test participants at the pre-job briefing. This team has been great to work with and together they really did an excellent job.”

Sandia pulls units randomly from the stockpile for such tests, as well as for lab tests on the weapon’s nonnuclear components. Flight tests drop units from aircraft the Tonopah Test Range in Nevada.

The nuclear package, removed prior to the tests, is studied separately by the design laboratory, either Los Alamos or Lawrence Livermore national laboratory. Lab tests study the nonnuclear components under different conditions. Environments are more controlled so researchers can closely measure how components function. Other laboratory tests examine components and materials for signs of aging by repeatedly subjecting them to various conditions.

As test preparations moved forward on Nov. 20, a radio on a table in the room crackled with updates: the rocket motors and explosives were wired; the test team pulled back to a control facility 5,000 feet away to arm the firing system remotely; the final countdown of 5-4-3-2-1. A camera that had been focused on the weapon panned down to the target at T minus 30 seconds to capture the weapon slamming through the concrete.

An unimaginable amount of detail and work goes into getting as much information as possible out of the tests. This one involved a series of calibration tests and qualifying reviews beforehand, dozens of people from Divisions 2000, 4000, and 1000, as well as researchers from Los Alamos who had components on the weapon and personnel helping set up and monitor the test. Some of the test team arrived to begin final preparations at 4 a.m. on test day. Sandia’s Emergency Operations and the Kirtland Air Force Base Fire Department were on hand when the test went off.

Afterward, the radio continued to snap out updates: no fires — a possibility from burning rocket fuel debris; no debris from the weapon around the target; levels of toxic gas from the burning rocket propellant at zero; the cable cutters all fired; in short, everything at the scene indicated the test was normal.

“Boring is good,” commented Justine.

The area was declared safe about 15 minutes after the weapon hit the target, and the test team moved in to begin moving cables and cleaning up the site. Two large cranes moved the target the next day to allow technicians to excavate the B61 in preparation for sending it for disassembly at the Pantex plant in Amarillo, where it was built and where it will be disassembled for analysis.

Quantifying the oxygen effect on hydrogen embrittlement

In the Hydrogen Effects on Materials Laboratory, Joe Ronevich, left, and Jeff Campbell (both 8252) use a crane to maneuver a stainless steel pressure vessel containing material test specimens into place before connecting it to the manifold piping and filling it with high-pressure hydrogen gas. (Photo by Dino Vournas)

Hydrogen embrittlement, the process by which metals become less ductile and more susceptible to fracture during exposure to hydrogen, is a problem that has vexed scientists for more than a century. As a small and highly mobile molecule, it seems that nothing can stop hydrogen from working its way into and degrading any structural material, especially high-strength materials — except perhaps hydrogen’s water partner, oxygen.

“In the materials science community, there has been some discussion that hydrogen embrittlement can be inhibited or even stopped when certain gas species are blended with the hydrogen, even at parts per million quantities,” says principal investigator Brian Somerday (8252). “Oxygen is one of those gas species.”

Brian and Joe Ronevich (8252) had the opportunity to put this theory to the test in the Hydrogen Effects on Materials Lab through a collaborative project with the US Army Armament Research, Development and Engineering Center (ARDEC), headquartered at Picatinny Arsenal, N.J. ARDEC is developing a next-generation artillery technology, the Combustion Light Gas Gun (CLGG), that propels projectiles through the combustion of hydrogen and oxygen.

The CLGG substitutes gaseous hydrogen and oxygen for traditional gun propellant. Combustion gases are significantly lighter than those used in a conventional gun, permitting considerable increases in muzzle velocity. This results in the potential for greatly extended range and about 11 times the coverage of conventional artillery. Using hydrogen as the primary propellant has other advantages, like environmentally benign steam as the combustion byproduct. Hydrogen can also be rendered harmless by venting the gas into the atmosphere, and the gas has the added advantage of dual use: as a gun propellant and a fuel to power military vehicles.

But with hydrogen comes hydrogen embrittlement, exacerbated by the high-strength materials of the CLGG. The wild card is the added presence of oxygen, a necessary component of ARDEC’s combustion scheme.

“This raises a fundamental science question,” says Brian. “Can the oxygen present mitigate the inevitable hydrogen embrittlement? And if so, can you quantify that effect? From a science perspective, we think what we are doing to define the boundaries and variables that promote this inhibition is unique.”

He was well-prepared when the call came from ARDEC. In a recent project for the DOE Fuel Cell Technologies Office, Brian has been working to quantify the mitigating effects of oxygen on hydrogen-assisted crack growth in steels for hydrogen pipelines. “We have some experience now in mixed-gas tests, as opposed to pure hydrogen, which we were able to apply immediately to ARDEC’s problem,” he says.

ARDEC represents a new type of customer for Sandia’s core capability in hydrogen embrittlement. This research originated in the nuclear weapons program, specifically for gas transfer systems components. Over the last 10 years, Sandia has successfully applied this expertise to the hydrogen energy arena. “This project applies that core capability to a dimension of hydrogen embrittlement that has not been explored in much depth,” says Brian.

The oxygen effect

In April, Joe conducted a series of experiments for ARDEC in the Hydrogen Effects on Materials Lab. In 100-hour tests, he compared the effects of pure hydrogen against hydrogen with 0.5 percent O2 on two different high-strength materials, martensitic steel and a nickel-based superalloy.

“In the samples tested in pure hydrogen, we saw what we expected — the propagation of cracks quite quickly,” says Joe. “In the samples tested in mixed gas, we still saw crack propagation, but there was a significant time delay. These results were intriguing because the time delay indicated that oxygen had some mitigating effect, but the hydrogen embrittlement was not absolutely inhibited.”

In the pure hydrogen, cracks developed in both the martensitic steel and nickel-based superalloy within minutes. In the mixed gas, cracks did not begin to develop in the martensitic steel for more than 94 hours and in the nickel based superalloy for over one hour.

In September, Joe ran another series of experiments on a different martensitic steel that was heat-treated to two distinct strength levels and subjected to a hydrogen/1 percent oxygen mixture for 100 hours.

“This was an interesting result,” says Joe. “We anticipated that doubling the concentration of oxygen would have a higher inhibiting effect, but that is not what we saw. In both pure hydrogen and the mixed gas with 1 percent oxygen, the time for crack initiation ranged from minutes to hours, depending on the steel strength and applied stress level. The mitigating effect of oxygen was most pronounced for the low-strength steel, in which the onset of crack initiation in the mixed gas was about 40 times longer compared to the pure hydrogen.”

Brian anticipates that a third series of experiments could start soon, this time looking at the effects of a hydrogen/oxygen mixture on high-strength materials with a cycling mechanical load.

“The delay in crack propagation that we saw could be specific to these high-strength structural materials,” says Brian. “Or, on a different class of structural materials, maybe the inhibition effect is absolute. These results inform our understanding and endorse the value of this avenue of research. We could be looking at the hydrogen embrittlement problem in a very different way.”

Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.